1. Field of Invention
The present invention relates to Modeling, Design, Analysis, Simulation, and Evaluation (MDASE) aspects of gyrocompassing in relation to Far-Target Location (FTL) systems. FTL systems provide an extensive support for many joint operations and efforts, which are critical for a variety of government uses. For instance, they provide a solid foundation for many military joint operations. The basic principle of gyrocompassing is based on the measurement of the earth's angular velocity vector and the local gravity force vector. The US Army needs a suite of effective Modeling, Analysis, Simulation, and Evaluation tools for the design, development, implementation and validation of the gyrocompass based azimuth and attitude determination modules for the FTL systems.
2. Description of Related Arts
There is a need for high accuracy azimuth information for man-portable Far-Target Location (FTL) systems. The largest source of Target Location Error (TLE) in the existing FTL systems is in “azimuth”. Target azimuth in today's man-portable target locator systems is determined using an embedded Digital Magnetic Compass (DMC). Though current DMCs provide better than 1° accuracy (<17.8 mils) in a benign environment, the accuracy in a tactical field environment is somewhat less due to the errors caused by nearby magnetic disturbances (e.g. vehicles, buildings, power lines, etc.) and local variations in the Earth's geo-magnetic field. In addition, DMCs require cumbersome calibration procedures.
Night Vision & Electronic Sensors Directorate is investing in component technologies for JETS via the Target Location & Designation System (TLDS) Advanced Technology Objective (ATO) number D.CER.2008.03. There is an intent-to-feed technology into the TLDS ATO, which in turn, will feed JETS. Accurate target azimuth (±1 mil) gyrocompass (GC)-IMU performance shall be achieved within 3 minutes of turn-on/initialization. The size, weight, and power of the objective GC-IMU module is to be made suitable for man-portable and/or hand-held FTL systems (<16 cubic inches, <2 pounds, and <3 watts respectively). Along with the technical parameters, GC-IMU setup and initialization procedures are to be evaluated against the constraints of the forward observer's mission. The GC-IMU module enables man-portable FTL systems (JETS) to achieve the ±1 mil target azimuth error required to “call-for-fire” when employing precision guided weapons. The specific intent is to research and demonstrate precision GC-IMU technology for the express purpose of eventual integration into a man-portable and/or hand-held FTL system (e.g. JETS).
The disclosed methods are to address the need for more accurate targeting information by determining the azimuth and vertical angle of the target by providing an azimuth accuracy of ±4 mils Probable Error (PE) threshold requirement (T) and ±1 mils PE objective requirement (O) between 60° north and south latitude. Likewise, the technical approach is to provide a vertical angle accuracy of ±4 mils PE (T) and ±1 mils PE (O) between 60° north and south latitude. The temporal stability of the accuracy is to be maintained for more than thirty (30) minutes (T)/more than sixty (60) minutes (O) after initialization. The approach requires no systematic system reinitialization (to maintain accuracy) within this time frame. The compensation, calibration and initialization methods are to be performed internally and without the use of an external computer. The accuracy is to be met while installed on a tripod and is to maintain required accuracy for the system under deterministic motion (sinking of the tripod) and random vibrations (due to wind). The technical approach is to provide for necessary Figure of Merit (FOM) on the azimuth angle and other angle outputs to indicate the quality of the angle estimates with 95% confidence. The proposed approach is to provide the required accuracy stated above within an initialization time of less than 240 sec (T)/90 sec (O) of the powering of the module. The technology demonstrator is to weigh less than 2 lbs (T)/0.2 lbs (O) and have volume of no more than 35 in3 (T)/0.25 in3 (O). The proposed approach is to provide required accuracy under the slew rate of 30 deg/sec (T)/360 deg/sec (O). The proposed approach is to provide required azimuth accuracy under the following orientation range: Pitch of ±500 mils (T)/±800 mils (O) and Bank of ±270 mils (T)/±500 mils (O). The proposed approach input power requirements are not to exceed 5 W (T)/3 W (O). The operating environment is to be from −40° C. to +70° C. The technical approach is required to meet the accuracy requirement after a shock of 40 g/11 ms. The technical approach is to meet the above requirements when integrated into a targeting system in a tripod mounted (T) or handheld (O) mode of operation. The technical approach is to meet the accuracy requirements in all weather conditions. The technical approach performance is not to degrade when electronically jammed or magnetic interfered. The technical approach is to meet the above requirements when operated under the battlefield environment (including but not limited to operations under trees, in forest, in urban canyon, nearby tanks, HMMWVs, buildings, and in the back of a pick-up truck).
In regard to gyrocompassing accuracy considerations critical gyrocompassing sensor parameters are the accelerometer and gyroscope bias errors and the gyro angle random walk (ARW) errors. The accelerometer and gyro bias leads to azimuth error:
      ϕ    D    =                    δω        E                    Ω        ⁢                                  ⁢                  Cos          ⁡                      (            L            )                                -                            δ          ⁢                                          ⁢                      a            E                          g            ⁢              tan        ⁡                  (          L          )                    
where L=latitude, φ=misalignments error in (N)orth, (E)ast, and (D)own, δωE=gyro error (bias)−in the NED coordinate frame, δaE=accelerometer error (bias) in the NED coordinate frame, Ω=earth's angular rate and g=gravity.
The gyro angle random walk versus the azimuth error is given by:
      ϕ    D    =      n          Ω      ⁢                          ⁢              Cos        ⁡                  (          L          )                    ⁢                        t          a                    
where n=random walk in degrees per square root hour, L=latitude, ta=averaging time and Ω=earth's rate.
The above relations show that to achieve an azimuth accuracy at 45° latitude within 90 sec of turn-on with an azimuth error bound of ˜4 mil the accelerometer bias must be 4 mG, the gyro bias instability 0.04°/hr and the Gyro Angle Random Walk (ARW) 0.004°/sq-rt hr. If the azimuth error bound is reduced to ˜1 mil the accelerometer bias must be 1 mG, the gyro bias instability 0.01°/hr and the Gyro Angle Random Walk (ARW) 0.001°/sq-rt hr.
Additional sensor parameters affecting gyrocompassing accuracy are error sources such as acceleration rate, scale factor errors, axis-misalignments, turn-on repeatability, turn-on bias stability, non-linearity, thermal effects etc. The combined impact of the error sources on gyrocompassing performance is not fully understood. The optimal trajectory and the optimal filtering have immense impact on the gyrocompassing performance. The trade study between the sensor error parameters versus the choice of system architecture needs to be modeled and understood. Additional considerations involve the impact of motion on the gyrocompass performance. Man-Portable applications are typically stationary which further constrains gyro performance. It is also noted that de-coupling and estimation of errors is easier in a high-dynamic application.
The interaction of error sources and their impact on the ultimate gyrocompassing accuracy are highly nonlinear and not possible to effect analytically. Thus, there is an essential need for a gyrocompassing simulation system that allows detailed parametric gyrocompassing performance evaluation based on error sources considered either in isolation or in any desired combination configuration.